Biogeochemical Processes and Marine Benthic Community Structure: Which Follows Which?

Abstract

To what extent do differences in sediment biogeochemistry determine which animals are present, and to what extent does the nature of the animal community control the rates and types of biogeochemical processes?Animals both can affect primary production, decomposition and element cycling in the sediments through direct and indirect mechanisms. Animals may: a) reduce primary production by overgrazing and depleting stocks of producers, but may also stimulate production indirectly by increasing rates of nutrient regeneration and recycling; b) alter decomposition directly by assimilating and breaking down organic matter in the sediments; c) affect biogeochemical processes indirectly by 1) irrigating sediments, which alters the concentration of oxygen, other electron acceptors and metabolites in pore waters, 2) redistributing organic matter in the sediments by bioturbation, 3) grazing microbes, 4) physically breaking up particles which changes their surface-to-volume ratio and exposes new surfaces for colonization by microbes, and 5) excreting nutrients.Overall rates of decomposition in the sediments appear to be only weakly affected by benthic animals. Thus we predict that changes in benthic community structure would have only a small effect on decomposition although the pathways of decomposition (aerobic vs. anaerobic) might shift. The converse does not appear to be true, however; we expect the structure of benthic communities to change in decomposition rates, with some communities being excluded from sediments with high rates of decomposition. In contrast microalgal primary production may be strongly controlled by the animal community. In many systems, grazing completely consumes the benthic algae, at least for a portion of the year.There is some evidence that the structure of the animal community alters sulfur cycling in the sediments. In laboratory experiments, the partitioning between oxic metabolism and sulfate reduction shifts with the type of animal community. Sulfur burial over geologic time has decreased, perhaps due to the evolution of deeper burrowing organisms. It also appears that changes in benthic community structure have a large potential to alter nitrogen dynamics, especially inorganic nitrogen release and denitrification. Because the nitrogen cycle is closely linked with primary production and carbon loading to the sediments, it provides feedback mechanisms between the fauna and other biogeochemical processes.